Generation and stability of bulk nanobubbles: A review and perspective

SONIC: A Speed of Sound Measurement for Nanobubble Characterization

Nanobubbles (NBs)─gas inclusions in water with diameters <1 μm─are of growing interest because of their unique properties and their potential for transformative applications. For example, it has been reported that NBs exist in water over long periods (i.e., weeks to months) and can act as free gas reservoirs. However, NBs are a source of scientific debate, particularly regarding characterization methods. Conventional methods, such as dynamic light scattering, nanoparticle tracking analysis, and nanoflow cytometry, cannot distinguish between nanoparticles and NBs since they are insensitive to the differences of the physical properties of the materials. However, acoustic (speed of sound) measurements can be used to quantify NBs because they rely on the compressibility dependence of gases (κgas) which is considerably larger than liquids (κwater) and solids. In the present work, a speed of sound measurement for nanobubble characterization (SONIC) was designed and developed to probe the compressibility variations diagnostic to NBs in water. NBs in water act as acoustic scatters that reduce the speed of sound relative to the bubble-free water. This decrease in the speed of sound can only be attributed to the existence of gas bubbles due to the strong compressibility dependence that solid nanoparticles lack. The results obtained from the acoustic measurements are compared with the observations from nanoparticle tracking analysis to confirm the existence of NBs in water. SONIC was validated in water with different molalities of NaCl (aq), and in the presence of solid nanoparticles of similar size and concentration to the NBs. SONIC is the first technique that addresses an important bottleneck of NB characterization by providing accurate and selective characterization of NBs in complex water mixtures that will help the behavior of NBs to be better understood and accelerate their application in many fields.

Ionic adsorption on bulk nanobubble interfaces and its uncertain role in diffusive stability

HYPOTHESIS

Bulk nanobubbles have been proposed to improve gas exchange in a variety of applications, such as in water treatment, theragnostics, and microfluidic surface cleaning. However, there is currently no consensus regarding the mechanism responsible for their reportedly long lifetimes, which contradicts classical understanding of diffusive bubble dynamics. Recently, there has been increasing support for an electrostatic stability mechanism, following from experiments that observe negatively charged zeta potentials around nanobubbles.

SIMULATIONS

We use high-fidelity Molecular Dynamics simulations to model bulk nanobubbles under mechanical equilibrium in a sodium iodide electrolyte solution, to investigate ionic adsorption on the liquid-gas interface, and resulting zeta potential. We critically examine the hypothesised electrostatic stress underpinning this previously suggested stability mechanism, which is theorised to stabilise the nanobubbles against dissolution by counteracting the otherwise dominant effects of surface tension, however, has been too difficult to directly measure in experiments.

FINDINGS

Ions adsorb onto the liquid-gas interface, confirming an Electric Double Layer (EDL) distribution around the nanobubble with an estimated zeta potential, in accordance with experiments. However, we find no significant electrostatic stress exerted on the nanobubble surface, as any ion charge density in the EDL is completely neutralised by the rearrangement of the water molecules. As a result, the internal gas pressure is still well predicted by the standard Laplace pressure equation (with a fitted Tolman length correction ), challenging an essential assumption underlying the previously proposed theories, and we instead speculate on alternative mechanisms for electrostatic-based stability.

Strategy to relieve cellular oxidative stress based on ultra-small nanobubbles without exogenous antioxidants

Reactive oxygen species (ROS) produced in living systems are essential to physiological processes. However, excess ROS in the organism (oxidative stress) damages crucial cell components, leading to many diseases. Although some commercial antioxidants can counteract ROS damage, their inadequate tissue penetration, disruption of normal ROS functions, and possible toxicity have led to disappointing results in clinical trials for ROS-induced chronic diseases. Thus, new antioxidant strategies are warranted. Herein, we report a novel "antioxidant" composed of pure nitrogen gas in an ultra-small nanobubble (UNB) form, which can relieve oxidative stress in cells. Our results indicate that UNBs can reduce cellular ROS levels under oxidative stress and increase survival and proliferation. Besides, UNBs can decrease the oxidative damage to cellular biomacromolecules (lipids, proteins, and nuclear acids). Thus, UNBs are a promising nonchemical antioxidative strategy with potential applications against oxidative stress-related diseases and without the natural defect of chemical antioxidants.

The Existence and Stability Mechanism of Bulk Nanobubbles: A Review

Since they were shown to be a potential phenomenon through experimentation, bulk nanobubbles (BNBs) have been a long-standing controversy. The controversy mainly originates from the fact that their stability cannot be well explained by the established theories. Although nanobubbles have been applied in many fields, the controversial stability issue has been a hanging "cloud" looming over the nanobubble research. This review focuses on why the stability of nanobubbles cannot be depicted by the current theories from thermodynamics and dynamics perspectives. Moreover, a number of current models pertaining to bulk nanobubble stability are compiled. It is anticipated that this review will give readers a better grasp of the current state of bulk nanobubble research and provide some insight for further studies in this area.

Assessment of Tamarix smyrnensis for Phytoremediation Capacity of Laterite Mine Spoils

The phytoremediation potential of the halophytic plant, Tamarix smyrnensis (T. smyrnensis), was examined in toxic metal spoils assisted by biochar and irrigation by air nanobubbles. The substrate (spoil) used in the present study was derived from areas close to laterite (Ni-containing ores) mines. The efficiency of biochar addition in two rates (5 t/ha and 20 t/ha) to improve microbial properties and stabilize soil aggregates was also examined. Furthermore, the effect of irrigation with air-nanobubble-supplemented water was evaluated for the remediation of toxic metal spoils. The physiological condition of the plant species was investigated in terms of biomass, height, chlorophyll content, and antioxidant enzymes. The alkali and heavy metal accumulation and their distribution in the plant parts were assessed to explore whether toxic metals could accumulate in the root and further translocate to the aboveground tissues. The growth of T. smyrnensis was not adversely affected by its cultivation in lateritic spoil, and the highest rate of biochar exhibited a beneficial effect on plant growth in terms of weight (aerial and subterranean biomass). The highest biochar application rate led to significant increases in total chlorophyll content, showing a 97.6% increase when biochar is used alone and a 136% increase when combined with nanobubble irrigation. Remarkably, only when combining irrigation with air nanobubbles and low biochar supplementation did the translocation of the metals from soil to the aboveground tissues occur as the translocation factor was estimated to be greater than unity (TF > 1). The bioconcentration factors remained below 1.0 (BCF < 1) across all treatments, demonstrating limited mobilization from soil to plant tissues despite the application of soil amendments. Finally, the application of nanobubbles increased slightly but not substantially the total uptake of metals, which showed a significant decrease compared to the control groups when the lower dosage of biochar was utilized.

Theoretical model of dynamics and stability of nanobubbles on heterogeneous surfaces

Surface nanobubbles have revealed a new mechanism of gas-liquid-solid interaction at the nanoscale; however, the nanobubble evolution on real substrates is still veiled, because the experimental observation of contact line motions at the nanoscale is too difficult.

HYPOTHESIS

This study proposes a theoretical model to describe the dynamics and stability of nanobubbles on heterogeneous substrates. It simultaneously considers the diffusive equilibrium of the liquid-gas interface and the mechanical equilibrium at the contact line, and introduces a surface energy function to express the substrate's heterogeneity.

VALIDATION

The present model unifies the nanoscale stability and the microscale instability of surface bubbles. The theoretical predictions are highly consistent to the nanobubble morphology on heterogeneous surfaces observed in experiments. As the nanobubbles grow, a lower Laplace pressure leads to weaker gas adsorption, and the mechanical equilibrium can eventually revert to the classical Young-Laplace equation above microscale.

FINDINGS

The analysis results indicate that both the decrease in substrate surface energy and the increase in gas oversaturation are more conducive to the nucleation and growth of surface nanobubbles, leading to larger stable sizes. The larger surface energy barriers result in the stronger pinning, which is beneficial for achieving stability of the pinned bubbles. The present model is able to reproduce the continual behaviors of the three-phase contact line during the nanobubble evolution, e.g., "pinning, depinning, slipping and jumping" induced by the nanoscale defects.

Efficacy of Air and Oxygen Micro-nano Bubble Waters Against Colletotrichum gloeosporioides and Impacts on Postharvest Quality of 'Fan Retief' Guava Fruit

This study focused on the application of micro-nano bubbles (MNBs) water generated using air or oxygen (O2), as an alternative to chlorine-based wash for fruits. For the in vitro and in vivo investigation, 106 spore or conidia/mL Colletotrichum gloeosporioides suspension was used, and treated with solutions of air- or O2-MNB for 30- or 60-min, sodium hypochlorite (NaOCl), and untreated (as control). In the second experiment, freshly harvested guava fruits were washed with tap water (control), NaOCl (standard practice), air-, or O2-MNB (for 15- or 30-min). All samples were packaged, stored for 21 days at 13 °C, and monitored for changes in natural microbial population and quality attributes. Based on the confocal laser and transmission electron microscopy results, exposure of C. gloeosporioides to air-MNB for 60 min resulted in the lowest viable cell count (%) compared to control and other treatments (O2-MNB and NaOCl). Air- and O2-MNB treatments damaged cellular structures, disrupted cell membrane integrity, and deformed hyphal morphology. Washing 'Fan Retief' guava (Psidium guajava L.) in air- or O2-MNB (for 15 and/or 30 min), better-retained tissue strength, delayed changes in color, and total soluble solid (TSS) content. Notably, MNB treatments were as effective as NaOCl washing and significantly reduced microbial load on fruit surface by ≥2 Log (p < 0.05). Micro-nano bubble water treatment offers a new paradigm for decontamination and preservation of guava fruit quality.

Intensification of quercetin nanobubble formulation and performance by multi-factor optimization and interaction analysis

The natural flavonoid Quercetin (QT) showed a potential for various health benefits, but its pharmaceutical applications are hindered by low solubility, permeability, and limited bioavailability. This research aimed to synthesize, develop and optimize polylactic acid co-glycolic acid (PLGA) nanobubbles using solvent evaporation method as a sustained delivery system for QT, thus improving stability and bioavailability. Through a four-factor, three-level Box Behnken Design, 29 experimental runs were carried out to optimize QT-PLGA nanobubbles. An optimized formulation consisted of 50 mg QT, 250 mg PLGA, and 1.89% w/v PVA. The nanobubbles displayed a particle size of 139.5 ± 6.24 nm, polydispersity index of 0.296 ± 0.19, and zeta potential of -23.0 ± 3.44 mV, with an entrapment efficiency of 59.24 ± 3.08%. Analysis through Fourier transform infrared spectroscopy, differential scanning calorimetry, and X-ray diffraction confirmed no drug-polymer interaction, while scanning electron microscopy revealed a uniform spherical nanoparticle. In vitro studies exhibited an excellent drug release, and stability studies showed no significant changes after one month. In vivo studies in rats demonstrated increased Cmax (3.03) and AUC0-t (5.84), indicating an improved sustained release and absorption. These findings underscored a potential of QT-loaded PLGA nanobubbles to enhance the drug kinetics and bioavailability, offering possibilities for targeted drug delivery and improved therapeutic outcomes.

Nanobubble Contrast Enhanced Ultrasound Imaging: A Review

Contrast-enhanced ultrasound is currently used worldwide with clinical indications in cardiology and radiology, and it continues to evolve and develop through innovative technological advancements. Clinically utilized contrast agents for ultrasound consist of hydrophobic gas microbubbles stabilized with a biocompatible shell. These agents are used commonly in echocardiography, with emerging applications in cancer diagnosis and therapy. Microbubbles are a blood pool agent with diameters between 1 and 10 μm, which precludes their use in other extravascular applications. To expand the potential use of contrast-enhanced ultrasound beyond intravascular applications, sub-micron agents, often called nanobubbles or ultra-fine bubbles, have recently emerged as a promising tool. Combining the principles of ultrasound imaging with the unique properties of nanobubbles (high concentration and small size), recent work has established their imaging potential. Contrast-enhanced ultrasound imaging using these agents continues to gain traction, with new studies establishing novel imaging applications. We highlight the recent achievements in nonlinear nanobubble contrast imaging, including a discussion on nanobubble formulations and their acoustic characteristics. Ultrasound imaging with nanobubbles is still in its early stages, but it has shown great potential in preclinical research and animal studies. We highlight unexplored areas of research where the capabilities of nanobubbles may offer new advantages. As technology advances, this technique may find applications in various areas of medicine, including cancer detection and treatment, cardiovascular imaging, and drug delivery.

Clinical Applications of Micro/Nanobubble Technology in Neurological Diseases

Nanomedicine, leveraging the unique properties of nanoparticles, has revolutionized the diagnosis and treatment of neurological diseases. Among various nanotechnological advancements, ultrasound-mediated drug delivery using micro- and nanobubbles offers promising solutions to overcome the blood-brain barrier (BBB), enhancing the precision and efficacy of therapeutic interventions. This review explores the principles, current clinical applications, challenges, and future directions of ultrasound-mediated drug delivery systems in treating stroke, brain tumors, neurodegenerative diseases, and neuroinflammatory disorders. Additionally, ongoing clinical trials and potential advancements in this field are discussed, providing a comprehensive overview of the impact of nanomedicine on neurological diseases.

CHARMM36 All-Atom Gas Model for Lipid Nanobubble Simulation

Lipid nanobubbles with different gas cores may integrate the biocompatibility of lipids, powerful physicochemical properties of nanobubbles, and therapeutic effects of gas molecules, which thus promote enormous biomedical applications such as ultrasound molecular imaging, gene/drug delivery, and gas therapy. In order for further more precise applications, the exact molecular mechanisms for the interactions between lipid nanobubbles and biological systems should be studied. Molecular dynamics (MD) simulation provides a powerful computational tool for this purpose. However, previous state-of-the-art MD simulations of free gas nanobubble/lipid nanobubble employed the vacuum as their gas cores, which is not suitable for studying the interactions between functional lipid nanobubbles and biological systems and revealing the biological roles of gas molecules. Hence, in this work, we developed and optimized the CHARMM36 all-atom gas parameters for six gases including N2, O2, H2, CO, CO2, and SO2, which accurately reproduced the gas density at different pressures as well as the spontaneous formation of gas nanobubbles. Subsequent applications of these gas parameters for lipid nanobubble simulations also reproduced the self-assembly process of the lipid nanobubble. We further developed a Python script to generate all-atom lipid nanobubble simulation systems, which was proven to be efficient for all-atom MD simulations of lipid nanobubbles and to be able to capture the exact dynamics of gas molecules at the gas-lipid and lipid-water interfaces of the lipid nanobubble. In summary, the all-atom gas models proposed in this work are suitable for simulating free gas nanobubbles and lipid nanobubbles, which are supposed to overcome the shortcomings of previous state-of-the-art MD simulations with the vacuum replacing the gas core and play key roles in revealing the molecular-level interactions between lipid nanobubbles and biological systems.

Investigating the Potential of CO2 Nanobubble Systems for Enhanced Oil Recovery in Extra-Low-Permeability Reservoirs

Carbon Capture, Utilization, and Storage (CCUS) stands as one of the effective means to reduce carbon emissions and serves as a crucial technical pillar for achieving experimental carbon neutrality. CO2-enhanced oil recovery (CO2-EOR) represents the foremost method for CO2 utilization. CO2-EOR represents a favorable technical means of efficiently developing extra-low-permeability reservoirs. Nevertheless, the process known as the direct injection of CO2 is highly susceptible to gas scrambling, which reduces the exposure time and contact area between CO2 and the extra-low-permeability oil matrix, making it challenging to utilize CO2 molecular diffusion effectively. In this paper, a comprehensive study involving the application of a CO2 nanobubble system in extra-low-permeability reservoirs is presented. A modified nano-SiO2 particle with pro-CO2 properties was designed using the Pickering emulsion template method and employed as a CO2 nanobubble stabilizer. The suitability of the CO2 nanobubbles for use in extra-low-permeability reservoirs was evaluated in terms of their temperature resistance, oil resistance, dimensional stability, interfacial properties, and wetting-reversal properties. The enhanced oil recovery (EOR) effect of the CO2 nanobubble system was evaluated through core experiments. The results indicate that the CO2 nanobubble system can suppress the phenomena of channeling and gravity overlap in the formation. Additionally, the system can alter the wettability, thereby improving interfacial activity. Furthermore, the system can reduce the interfacial tension, thus expanding the wave efficiency of the repellent phase fluids. The system can also improve the ability of CO2 to displace the crude oil or water in the pore space. The CO2 nanobubble system can take advantage of its size and high mass transfer efficiency, among other advantages. Injection of the gas into the extra-low-permeability reservoir can be used to block high-gas-capacity channels. The injected gas is forced to enter the low-permeability layer or matrix, with the results of core simulation experiments indicating a recovery rate of 66.28%. Nanobubble technology, the subject of this paper, has significant practical implications for enhancing the efficiency of CO2-EOR and geologic sequestration, as well as providing an environmentally friendly method as part of larger CCUS-EOR.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability

The origin of the stability of bulk Nanobubbles (NBs) has been the object of scrutiny in recent years. The interplay between the surface charge on the NBs and the Laplace pressure resulting from the surface tension at the solvent-NB interface has often been evoked to explain the stability of the dispersed NBs. While the Laplace pressure is well understood in the community, the nature of the surface charge on the NBs has remained obscure. In this work, we aim to show that the solvent and the present ions can effectively polarize the NB surface by inducing a dipole moment, which in turn controls the NB stability. We show that the polarizability of the dispersed gas and the polarity of the dispersing solvent control the dipole-induced dipole interactions between the solvent and the NBs, and that, in turn, determines their stability in solution.

Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water

It has been widely recognized that the pH environment influences the nanobubble dynamics and hydroxide ions adsorbed on the surface may be responsible for the long-term survival of the nanobubbles. However, understanding the distribution of hydronium and hydroxide ions in the vicinity of a bulk nanobubble surface at a microscopic scale and the consequent impact of these ions on the nanobubble behavior remains a challenging endeavor. In this study, we carried out deep potential molecular dynamics simulations to explore the behavior of a nitrogen nanobubble under neutral, acidic, and alkaline conditions and the inherent mechanism, and we also conducted a theoretical thermodynamic and dynamic analysis to address constraints related to simulation duration. Our simulations and theoretical analyses demonstrate a trend of nanobubble dissolution similar to that observed experimentally, emphasizing the limited dissolution of bulk nanobubbles in alkaline conditions, where hydroxide ions tend to reside slightly farther from the nanobubble surface than hydronium ions, forming more stable hydrogen bond networks that shield the nanobubble from dissolution. In acidic conditions, the hydronium ions preferentially accumulating at the nanobubble surface in an orderly manner drive nanobubble dissolution to increase the entropy of the system, and the dissolved nitrogen molecules further strengthen the hydrogen bond networks of systems by providing a hydrophobic environment for hydronium ions, suggesting both entropy and enthalpy effects contribute to the instability of nanobubbles under acidic conditions. These results offer fresh insights into the double-layer distribution of hydroxide and hydronium near the nitrogen-water interface that influences the dynamic behavior of bulk nanobubbles.

Nanobubbles improve peroxymonosulfate-based advanced oxidation: High efficiency, low toxicity/cost, and novel collaborative mechanism

Cl- activated peroxymonosulfate (PMS) oxidation technology can effectively degrade pollutants, but the generation of chlorinated disinfection byproducts (DBPs) limits the application of this technology in water treatment. In this study, a method of nanobubbles (NBs) synergistic Cl-/PMS system was designed to try to improve this technology. The results showed the synergistic effects of NBs/Cl-/PMS were significant and universal while its upgrade rate was from 12.89% to 34.97%. Moreover, the synergistic effects can be further improved by increasing the concentration and Zeta potential of NBs. The main synergistic effects of NBs/Cl-/PMS system were due to the electrostatic attraction of negatively charged NBs to Na+ from NaCl, K+ from PMS, and H+ from phenol, which acted as a "bridge" between Cl- and HSO5- as well as phenol and Cl-/HSO5-, increasing active substance concentration. In addition, the addition of NBs completely changed the oxidation system of Cl-/PMS from one that increases environmental toxicity to one that reduces it. The reason was that the electrostatic attraction of NBs changed the active sites and degradation pathway of phenol, greatly reducing the production of highly toxic DBPs. This study developed a novel environmentally friendly oxidation technology, which provides an effective strategy to reduce the generation of DBPs in the Cl-/PMS system.

Advancing nanobubble technology for carbon-neutral water treatment and enhanced environmental sustainability

Gaseous nanobubbles (NBs) with dimensions ranging from 1 to 1000 nm in the liquid phase have garnered significant interest due to their unique physicochemical characteristics, including specific surface area, low internal gas pressure, long-term stability, efficient mass transfer, interface potential, and free radical production. These remarkable properties have sparked considerable attention in the scientific community and industries alike. These hold immense promise for environmental applications, especially for carbon-neutral water remediation. Their long-lasting stability in aqueous systems and efficient mass transfer properties make them highly suitable for delivering gases in the vicinity of pollutants. This potential has prompted research into the use of NBs for targeted delivery of gases in contaminated water bodies, facilitating the degradation of harmful substances and advancing sustainable remediation practices. However, despite significant progress in understanding NBs physicochemical properties and potential applications, several challenges and knowledge gaps persist. This review thereby aims to summarize the current state of research on NBs environmental applications and potential for remediation. By discussing the generation processes, mechanisms, principles, and characterization techniques, it sheds light on the promising future of NBs in advancing environmental sustainability. It explores their role in improving oxygenation, aeration, and pollutant degradation in water systems. Finally, the review addresses future research perspectives, emphasizing the need to bridge knowledge gaps and overcome challenges to unlock the full potential of this frontier technology for enhanced environmental sustainability.

Bulk Nanobubbles through Gas Supersaturation Originated by Hot and Cold Solvent Mixing

The nucleation mechanism of bulk nanobubbles remains unclear despite the considerable attention they have received in recent years. We propose two hypotheses: (i) The gas supersaturation in the bulk liquid is the primary factor for nanobubble nucleation, and (ii) the mixing of the same solvent at varying gas solubilities should produce nanobubbles, provided that the first hypothesis is correct. To test this hypothesis, we performed extensive experiments on nanobubble nucleation in both water and organic solvents. The temperature difference between hot and cold samples ranged from 10 to 80 °C in pure solvents such as water, methanol, ethanol, propanol, and butanol prepared and mixed in equal proportions. To the best of our knowledge, we report bulk nanobubble nucleation by mixing hot and cold solvents for the first time. The refractive index value calculations using Mie scattering theory confirmed the existence of nanobubbles. When surface tension dominates over surface charge, the critical work for nanobubble formation is ΔFc ∝ 1/ξ2, and when surface charge dominates over surface tension, the critical work is ΔFc ∝ ξ1/4. Our experimental results verify such dependency by measuring nanobubbles nucleated with varying degrees of gas supersaturation.

Fluorocarbon Nanodroplets: Their Formation and Stability in Complex Solution Systems

Perfluorocarbon (PFC) nanodroplets (NDs) are expanding in a wide range of applications in biotechnology and nanotechnology. Their efficacy in biological systems is significantly influenced by their size uniformity and stability within bioelectrolyte contexts. Presently, methods for creating monodisperse, highly concentrated, and well-stabilized PFC NDs under harsh conditions using low energy consumption methods have not been thoroughly developed, and their stability has not been sufficiently explored. This gap restricts their applicability for advanced medical interventions in tissues with high pH levels and various electrolytic conditions. To tackle these challenges and to circumvent potential toxicity from surface stabilizers, we have conducted an in-depth investigation into the formation and stability of uncoated perfluorohexane (PFH) NDs, which were synthesized by using a low-energy consumption solvent exchange technique, across complex electrolyte compositions or a broad spectrum of pH levels. The results indicated that low concentrations of low-valent electrolyte ions facilitate the nucleation of NDs and consistently accelerate Ostwald ripening over an extended period. Conversely, high concentrations of highly valent electrolyte ions inhibit nucleation and decelerate the ripening process over time. Given the similarities between the properties of NDs and nanobubbles, we propose a potential stabilization mechanism. Electrolytes influence the Ostwald ripening of NDs by adjusting the adsorption and distribution of ions on the NDs' surface, modifying the thickness of the electric double layer, and fine-tuning the energy barrier between droplets. These insights enable precise control over the stability of PFC NDs through the meticulous adjustment of the surrounding electrolyte composition. This offers an effective preparation method and a theoretical foundation for employing bare PFC NDs in physiological settings.

High throughput microfluidic nanobubble generation by microporous membrane integration and controlled bubble shrinkage

Microfluidics has recently been proposed as a viable method for producing bulk nanobubbles for use in various applications. The portability, compact size, and capacity to precisely control fluids on a small scale are a few of the benefits of microfluidics that may be exploited to create customized bulk nanobubbles. However, despite the potential of microfluidic nanobubble generation, low throughput and limited nanobubble concentration remain challenging for microfluidics. Here, we integrate a microporous silicon membrane into a polydimethylsiloxane microfluidic chip to generate bulk nanobubbles in the 100-140 nm diameter range with a concentration of up to 108 mL-1. We investigate the nanobubble size and morphology using several characterisation techniques, including transmission electron microscopy, resonance mass measurement, dynamic light scattering, and the Tyndall effect. This new nanobubble generation technique can increase nanobubble concentration by ∼ 23 times compared to earlier microfluidic nanobubble generation platforms, which should increase the feasibility of translation to medical applications.

Unlocking the Potential of Nanobubbles: Achieving Exceptional Gas Efficiency in Electrogeneration of Hydrogen Peroxide

The electrogeneration of hydrogen peroxide (H2 O2 ) via the oxygen reduction reaction is a crucial process for advanced water treatment technologies. While significant effort is being devoted to developing highly reactive materials, gas provision systems used in these processes are receiving less attention. Here, using oxygen nanobubbles to improve the gas efficiency of the electrogeneration of H2 O2 is proposed. Aeration with nanobubbles is compared to aeration with macrobubbles under an identical experimental set-up, with nanobubbles showing a much higher gas-liquid volumetric mass transfer coefficient (KL a) of 2.6 × 10-2 min-1 compared to 2.7 × 10-4 min-1 for macrobubbles. Consequently, nanobubbles exhibit a much higher gas efficiency using 60% of O2 delivered to the system compared to 0.19% for macrobubbles. Further, it is observed that the electrogeneration of H2 O2 using carbon felt electrodes is enhanced using nanobubbles. Under the same dissolved oxygen levels, nanobubbles boost the reaction yield to 84%, while macrobubbles yield only 53.8%. To the authors' knowledge, this is the first study to investigate the use of nanobubbles in electrochemical reactions and demonstrate their ability to enhance gas efficiency and electrocatalytic response. These findings have important implications for developing more efficient chemical and electrochemical processes operating under gas-starving systems.

From Nanovesicles to Nanobubbles Based on Repeated Compression Method

Nanobubbles have been increasingly applied in biomedicine, which is attributed to their ability to work as ultrasound imaging contrast agents and powerful gene/drug carriers. Different production techniques or approaches have been developed to generate uniform and stable shelled nanobubbles. However, these shelled nanobubbles are usually prepared based on disordered shell materials, such as free phospholipids and polymers. In recent years, the continuous repeated compression method for a gas-liquid mixture has been developed to produce free and lipid-shelled nanobubbles. In this study, to explore the response of well-organized nanostructures to this method, the repeated compression method was used to treat preprepared liposomes and polymeric nanovesicles. Size distribution, morphologies, and ultrasound image contrast enhancement of these nanovesicles were determined before and after repeated compression. Results demonstrate that the presence of a phospholipid bilayer is vital to form liposome-based nanobubbles. And the low elastic modulus of the polymeric membrane is key to encapsulate gases into polymeric nanovesicles. Overall, it demonstrated the advantages of well-organized nanostructures to produce nanobubble structures, giving new insights into the preparation and understanding of nanobubbles.

Additive-Free Method for Enhancing the Volume Phase Transition Rate in Light-Responsive Hydrogels: A Study of Micro-Nano Bubble Water on PNIPAM-co-AAc Hydrogels

Light-responsive hydrogels containing light-thermal convertible pigments have received interest for their possible applications in light-responsive shutters, valves, drug delivery systems, etc. However, their utility is limited by the slow response time. In this study, we investigated the use of micro-nano bubble water as a preparation solvent to accelerate the volume phase transition kinetics of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AAc) hydrogels. The hydrogels were characterized by dynamic light scattering (DLS) and dissolved oxygen (DO) measurements. The mechanical properties, surface morphology, and chemical composition of the hydrogels were analyzed by Young's modulus measurements, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy, respectively. The results showed that hydrogels prepared with bubble water changed the volume transition rate by more than two orders of magnitude by simply changing the standing time of the bubble water for only a few hours. The cooperative diffusion coefficients obtained from the light-induced volume transition kinetics correlated linearly with Young's modulus and metastable state swelling ratio. Our results suggest that bubbles act as efficient water channels, thereby modulating the response rate and providing a simple, additive-free method for preparing hydrogels with a wide range of response rates.

Imaging Single H2 Nanobubbles Using Off-Axis Dark-Field Microscopy

A robust and detailed physicochemical description of electrochemically generated surface nanobubbles and their effects on electrochemical systems remains at large. Herein, we report the development and utilization of an off-axis, dark-field microscopy imaging tool for probing the dynamic process of generating single H2 nanobubbles at the surface of a carbon nanoelectrode. A change in the direction of the incident light is made to significantly reduce the intensity of the background light, which enables us to image both the nanoelectrode and nanobubble on the electrode surface or the metal nanoparticles in the vicinity of the electrode. The correlated electrochemical and optical response provides novel insights regarding bubble nucleation and dissolution on a nanoelectrode previously unattainable solely from its current-voltage response.

Superior performance of a membrane bioreactor through innovative in-situ aeration and structural optimization using computational fluid dynamics

The optimization of membrane bioreactors (MBRs) involves a critical challenge in structural design for mitigation of membrane fouling. To address this issue, a three-dimensional computational fluid dynamics (CFD) model was utilized in this study to simulate the hydrodynamic characteristics of a flat sheet (FS) MBR. The optimization of the membrane module configuration and operating conditions was performed by investigating key parameters that altered the shear stress and liquid velocity. The mixed liquor suspended solids (MLSS) concentration was found to increase the shear stress, leading to a more uniform distribution of shear stress. By optimizing the appropriate bubble diameter to 5 mm, the shear stress on the membrane surface was optimized with relatively uniform distribution. Additionally, extending the side baffle length dramatically improved the uniformity of the shear stress distribution on each membrane. A novel in-situ aeration method was also discovered to promote turbulent kinetic energy by 200 times compared with traditional aeration modes, leading to a more uniform bubble streamline. As a result, the novel in-situ aeration method demonstrated superior membrane antifouling potential in the MBR. This work provides a new approach for the structural design and optimization of MBRs. The innovative combination of the CFD model, optimization techniques, and novel in-situ aeration method has provided a substantial contribution to the advancement of membrane separation technology in wastewater treatment.

Synergism of Surfactant Mixture in Lowering Vapor-Liquid Interfacial Tension

Through employing molecular dynamics, in this work, we study how a two-component surfactant mixture cooperatively reduces the interfacial tension of a flat vapor-liquid interface. Our simulation results show that in the presence of a given insoluble surfactant, adding a secondary surfactant would either further reduce interfacial tension, indicating a positive synergistic effect, or increase the interfacial tension instead, indicating a negative synergistic effect. The synergism of the surfactant mixture in lowering surface tension is found to depend strongly on the structure complementary effect between different surfactant components. The synergistic mechanisms are then interpreted with minimization of the bending free energy of the composite surfactant monolayer via cooperatively changing the monolayer spontaneous curvature. By roughly describing the monolayer spontaneous curvature with the balanced distribution of surfactant heads and tails, we confirm that the positive synergistic effect in lowering surface tension is featured with the increasingly symmetric head-tail distributions, while the negative synergistic effect is featured with the increasingly asymmetric head-tail distributions. Furthermore, our simulation results indicate that minimal interfacial tension can only be observed when the spontaneous curvature is nearly zero.

Nanoparticles in Medicine: Current Status in Cancer Treatment

Cancer is still a leading cause of deaths worldwide, especially due to those cases diagnosed at late stages with metastases that are still considered untreatable and are managed in such a way that a lengthy chronic state is achieved. Nanotechnology has been acknowledged as one possible solution to improve existing cancer treatments, but also as an innovative approach to developing new therapeutic solutions that will lower systemic toxicity and increase targeted action on tumors and metastatic tumor cells. In particular, the nanoparticles studied in the context of cancer treatment include organic and inorganic particles whose role may often be expanded into diagnostic applications. Some of the best studied nanoparticles include metallic gold and silver nanoparticles, quantum dots, polymeric nanoparticles, carbon nanotubes and graphene, with diverse mechanisms of action such as, for example, the increased induction of reactive oxygen species, increased cellular uptake and functionalization properties for improved targeted delivery. Recently, novel nanoparticles for improved cancer cell targeting also include nanobubbles, which have already demonstrated increased localization of anticancer molecules in tumor tissues. In this review, we will accordingly present and discuss state-of-the-art nanoparticles and nano-formulations for cancer treatment and limitations for their application in a clinical setting.

Bubble-Based Drug Delivery Systems: Next-Generation Diagnosis to Therapy

Current radiologic and medication administration is systematic and has widespread side effects; however, the administration of microbubbles and nanobubbles (MNBs) has the possibility to provide therapeutic and diagnostic information without the same ramifications. Microbubbles (MBs), for instance, have been used for ultrasound (US) imaging due to their ability to remain in vessels when exposed to ultrasonic waves. On the other hand, nanobubbles (NBs) can be used for further therapeutic benefits, including chronic treatments for osteoporosis and cancer, gene delivery, and treatment for acute conditions, such as brain infections and urinary tract infections (UTIs). Clinical trials are also being conducted for different administrations and utilizations of MNBs. Overall, there are large horizons for the benefits of MNBs in radiology, general medicine, surgery, and many more medical applications. As such, this review aims to evaluate the most recent publications from 2016 to 2022 to report the current uses and innovations for MNBs.

Microfluidic nanobubbles: observations of a sudden contraction of microbubbles into nanobubbles

Microfluidic devices are often utilized to generate uniform-size microbubbles. In most microfluidic bubble generation experiments, once the bubbles are formed the gas inside the bubbles begin to dissolve into the surrounding aqueous environment. The bubbles shrink until they attain an equilibrium size dictated by the concentration and type of amphiphilic molecules stabilizing the gas-liquid interface. Here, we exploit this shrinkage mechanism, and control the solution lipid concentration and microfluidic geometry, to make monodisperse bulk nanobubbles. Interestingly, we make the surprising observation of a critical microbubble diameter above and below which the scale of bubble shrinkage dramatically changes. Namely, microbubbles generated with an initial diameter larger than the critical diameter shrinks to a stable diameter that is consistent with previous literature. However, microbubbles that are initially smaller than the critical diameter experience a sudden contraction into nanobubbles whose size is at least an order-of-magnitude below expectations. We apply electron microscopy and resonance mass measurement methods to quantify the size and uniformity of the nanobubbles, and probe the dependence of the critical bubble diameter on the lipid concentration. We anticipate that further analysis of this unexpected microbubble sudden contraction regime can lead to more robust technologies for making monodisperse nanobubbles.

An antioxidation strategy based on ultra-small nanobubbles without exogenous antioxidants

Antioxidation is in demand in living systems, as the excessive reactive oxygen species (ROS) in organisms lead to a variety of diseases. The conventional antioxidation strategies are mostly based on the introduction of exogenous antioxidants. However, antioxidants usually have shortcomings of poor stability, non-sustainability, and potential toxicity. Here, we proposed a novel antioxidation strategy based on ultra-small nanobubbles (NBs), in which the gas-liquid interface was employed to enrich and scavenge ROS. It was found that the ultra-small NBs (~ 10 nm) exhibited a strong inhibition on oxidization of extensive substrates by hydroxyl radicals, while the normal NBs (~ 100 nm) worked only for some substrates. Since the gas-water interface of the ultra-small NBs is non-expendable, its antioxidation would be sustainable and its effect be cumulative, which is different to that using reactive nanobubbles to eliminate free radicals as the gases are consumptive and the reaction is unsustainable. Therefore, our antioxidation strategy based on ultra-small NB would provide a new solution for antioxidation in bioscience as well as other fields such as materials, chemical industry, food industry, etc.

On Nanobubble Dynamics under an Oscillating Pressure Field during Salting-out Effects and Its DLVO Potential

We have investigated the origin, stability, and nanobubble dynamics under an oscillating pressure field followed by the salting-out effects. The higher solubility ratio (salting-out parameter) of the dissolved gases and pure solvent nucleates nanobubbles during the salting-out effect, and the oscillating pressure field enhances the nanobubble density further as solubility varies linearly with gas pressure by Henry's law. A novel method for refractive index estimation is developed to differentiate nanobubbles and nanoparticles based on the scattering intensity of light. The electromagnetic wave equations have been numerically solved and compared with the Mie scattering theory. The scattering cross-section of the nanobubbles was estimated to be smaller than the nanoparticles. The DLVO potentials of the nanobubbles predict the stable colloidal system. The zeta potential of nanobubbles varied by generating nanobubbles in different salt solutions, and it is characterized by particle tracking, dynamic light scattering, and cryo-TEM. The size of nanobubbles in salt solutions was reported to be higher than that in pure water. The novel mechanical stability model is proposed by considering both ionic cloud and electrostatic pressure at the charged interface. The ionic cloud pressure is derived by electric flux balance, and it is found to be twice the electrostatic pressure. The mechanical stability model for a single nanobubble predicts the existence of stable nanobubbles in the stability map.

Nanobubble technologies: Applications in therapy from molecular to cellular level

Nanobubbles are gaseous entities suspended in bulk liquids that have widespread beneficial usage in many industries. Nanobubbles are already proving to be versatile in furthering the effectiveness of disease treatment on cellular and molecular levels. They are functionalized with biocompatible and stealth surfaces to aid in the delivery of drugs. At the same time, nanobubbles serve as imaging agents due to the echogenic properties of the gas core, which can also be utilized for controlled and targeted delivery. This review provides an overview of the biomedical applications of nanobubbles, covering their preparation and characterization methods, discussing where the research is currently focused, and how they will help shape the future of biomedicine.

Do Ultrafine Bubbles Work as Oxygen Carriers?

Fine bubbles (FBs) are bubbles with sizes less than 100 μm and are divided into ultrafine bubbles (UFBs, < 1 μm) and microbubbles (MBs, 1-100 μm) depending on their size. Although FB aeration is known as a more efficient way than macrobubble aeration to increase the oxygen level in unoxygenated water, few reports have demonstrated whether dispersed UFBs work as oxygen carriers or not. Furthermore, oxygen supersaturation is one of the attractive characteristics of FB dispersion, but the reason is yet to be revealed. In this study, we evaluated the relationship between the FBs, especially UFB concentration, and oxygen content in several situations to reveal the two questions. The FB concentration and oxygen content were examined using particle analyzers and our developed oxygen measurement method, which can measure the oxygen content in FB dispersion, respectively. First, in the evaluations of the oxygen dispersion from UFBs with respect to the surrounding oxygen level, UFBs did become neither small nor diminish even in degassed water. Second, the changes in UFBs and oxygen content upon storage temperature and the existence of a lid during storage were evaluated, and there was no correlation between them. It means UFBs contribute little to the oxygen content in UFB dispersion. Furthermore, the oxygen content in the UFB dispersion decreased over time identically as that of the oxygen-supersaturated water with little UFBs. Third, we evaluated the relationship between FB concentration and oxygen content during FB generation by measuring them simultaneously. The results showed that dispersed MB and UFB concentrations did not account for the supersaturation of the FB dispersion. From the result, it was revealed that 100-200 nm of UFBs themselves did not work as oxygen carriers, and the oxygen supersaturation in FB dispersions was due to the supersaturated state of dissolved oxygen that was prepared during the FB generation process.

Different nanobubbles mitigate cadmium toxicity and accumulation of rice (Oryza sativa L.) seedlings in hydroponic cultures

Cadmium (Cd) contamination can pose a severe threat to food production and human health. The accumulation of Cd in rice will decrease rice biomass, photosynthetic activity, and antioxidant capacity, affecting crop yield. The effects of different nanobubbles on the growth and Cd accumulation of rice seedlings under hydroponic conditions were investigated in this study. The results showed that the biomass, photosynthetic pigment content, and antioxidant enzyme activity of rice seedlings decreased when treated with Cd alone and that Cd induced lipid peroxidation in rice seedlings. However, when different types of nanobubbles were introduced into the nutrient solution, the bioavailability of Cd in the solution was reduced. As a result, the Cd content in rice was significantly decreased compared to treatment with Cd alone. Nanobubbles increased the biomass of rice, enhanced photosynthesis, and improved the antioxidant capacity of rice by increasing antioxidant enzyme activities to alleviate Cd-induced oxidative stress. At the same time, nanobubbles increased the Fe content in rice, which decreased the Cd content, as Cd is antagonistic to Fe. In conclusion, these results suggested that nanobubbles are a potential method of mitigating Cd stress that may help to improve rice yield and could be further explored in production.

Ostwald ripening of aqueous microbubble solutions

Bubble solutions are of growing interest because of their various technological applications in surface cleaning, water treatment, and agriculture. However, their physicochemical properties, such as the stability and interfacial charge of bubbles, are not fully understood yet. In this study, the kinetics of radii in aqueous microbubble solutions are experimentally investigated, and the results are discussed in the context of Ostwald ripening. The obtained distributions of bubble radii scaled by mean radius and total number were found to be time-independent during the observation period. Image analysis of radii kinetics revealed that the average growth and shrinkage speed of each bubble is governed by diffusion-limited Ostwald ripening, and the kinetic coefficient calculated using the available physicochemical constants in the literature quantitatively agrees with the experimental data. Furthermore, the cube of mean radius and mean volume exhibit a linear time evolution in agreement with the Lifshitz-Slezov-Wagner (LSW) theory. The coefficients are slightly larger than those predicted using the LSW theory, which can be qualitatively explained by the effect of finite volume fraction. Finally, the slowdown and pinning of radius in the shrinkage dynamics of small microbubbles are discussed in detail.

Controllable generation of interfacial gas structures on the graphite surface by substrate hydrophobicity and gas oversaturation in water

Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.

Analysis of Gas Nanoclusters in Water Using All-Atom Molecular Dynamics

The Young-Laplace equation suggests that nanosized gas clusters would dissolve under the effects of perturbation. The fact that nanobubbles are observed raises questions as to the mechanism underlying their stability. In the current study, we used all-atom molecular dynamics simulations to investigate the gas-water interfacial properties of gas clusters. We employed the instantaneous coarse-graining method to define the fluctuating boundaries and analyze the deformation of gas clusters. Fourier transform analysis of the cluster morphology revealed that the radius and morphology deformation variations exhibit power law relationships with the vibrational frequency, indicating that the surface energy dissipated through morphology variations. Increasing pressure in the liquid region was found to alter the network of water molecules at the interface, whereas increasing pressure in the gas region did not exhibit this effect. The overall gas concentration was oversaturated and proportional to the gas density inside the clusters. However, the result of comparison with Henry's law reveals that the gas pressure at the interface reduced by the interfacial effects is much lower than that inside the gas region, thus reducing the demanding degree of oversaturation. Originating from the interfacial charge allocation, the magnitude of the electrostatic stress is greater than that of the gas pressure inside the cluster. However, the magnitude of the reversed tension induced by electrostatic stress is far below the value of interfacial tension. The potential of mean force (PMF) profiles revealed that a barrier potential at the interface hindered gas particles from escaping the cluster. Several effects contribute to stabilizing the gas clusters in water, including high-frequency morphological deformation, electrostatic stress, reduced interfacial tension, and gas oversaturation conditions. Our results suggest that gas clusters can exist in water under gas oversaturation conditions in the absence of hydrophobic contaminants or pinning charges at interfaces.

Effect of air nanobubbles on oxygen transfer, oxygen uptake, and diversity of aerobic microbial consortium in activated sludge reactors

Nanobubbles have the potential to curtail the loss of oxygen during activated sludge aeration due to their extensive surface areas and lack of buoyance in solution. In this study, nanobubble aeration was explored as a novel approach to enhance aerobic activated sludge treatment and benchmarked against coarse bubble aeration at the lab scale. Nanobubble aerated activated sludge reactors achieved greater dissolved oxygen levels at faster rates. Higher soluble chemical oxygen demand removal by 10% was observed when compared to coarse bubble aeration with the same amount of air. The activated sludge produced compact sludge yielding easier waste sludge for subsequent sludge handling. The samples showed fewer filamentous bacteria with a lower relative abundance of floc forming Corynebacterium, Pseudomonas, and Zoogloea in the sludge. The microbiome of the nanobubble-treated activated sludge showed significant shifts in the abundance of community members at the genus level and significantly lower alpha and beta diversities.

Generation methods, stability, detection techniques, and applications of bulk nanobubbles in agro-food industries: a review and future perspective

Nanobubble (NB) technologies have received considerable attention for various applications due to their low cost, eco-friendliness, scale-up potential, process control, and unique physical characteristics. NB stands for nanoscopic gaseous cavities, typically <1 μm in diameter. NBs can exist on surfaces (surface or interfacial NBs) and be dispersed in a bulk liquid phase (bulk NBs). Compared to the microbubbles, NBs exhibit high specific surface area, negative surface charge, and better adsorption. Bulk NBs can be generated by hydrodynamic/acoustic cavitation, electrolysis, water-solvent mixing, nano-membrane filtration, and so on. NBs exhibit extraordinary longevity compared to microbubbles, prompting the interest of the scientific community aiming for potential applications including medicine, agriculture, food, wastewater treatment, surface cleaning, and so on. Based on the limited amount of research work available regarding the influence of NBs on food matrices, further research, however, needs to be done to provide more insights into its applications in food industries. This review provides an overview of the generation methods for NBs, techniques to evaluate them, and a discussion of their stability and several applications in various fields of science were discussed. However, recent studies have revealed that, despite the many benefits of NB technologies, several NB generating approaches are still limited in their application in specific agro-food industries. Further study should focus on process optimization, integrating various NB generation techniques/combining with other emerging technologies in order to achieve rapid technical progress and industrialization of NB-based technologies.HighlightsNanobubbles (NBs) are stable spherical entities of gas within liquid and are operationally defined as having diameters less than 1 µm.Currently, various reported theories still lack the ability to explain the evidence and stability of NBs in water, numerous NB applications have emerged due to the unique properties of NBs.NB technologies can be applied to various food and dairy products (e.g. yogurt and ice cream) and other potential applications, including agriculture (e.g. seed germination and plant growth), wastewater treatment, surface cleaning, and so on.

Recent Developments in Generation, Detection and Application of Nanobubbles in Flotation

This paper reviews recent developments in the fundamental understating of ultrafine (nano) bubbles (NBs) and presents technological advances and reagent types used for their generation in flotation. The generation of NBs using various approaches including ultrasonication, solvent exchange, temperature change, hydrodynamic cavitation, and electrolysis was assessed. Most importantly, restrictions and opportunities with respect to the detection of NBs were comprehensively reviewed, focusing on various characterization techniques such as the laser particle size analyzer (LPSA), nanoparticle tracking (NTA), dynamic light scattering (DLS), zeta-phase light scattering (ZPALS), and zeta sizer. As a key feature, types and possible mechanisms of surfactants applied to stabilize NBs were also explored. Furthermore, flotation-assisted nano-bubbles was reported as an efficient method for recovering minerals, with a special focus on flotation kinetics. It was found that most researchers reported the existence and formation of NBs by different techniques, but there is not enough information on an accurate measurement of their size distribution and their commonly used reagents. It was also recognized that a suitable method for generating NBs, at a high rate and with a low cost, remains a technical challenge in flotation. The application of hydrodynamic cavitation based on a venturi tube and using the LPSA and NTA in laboratory scales were identified as the most predominant approaches for the generation and detection of NBs, respectively. In this regard, neither pilot- nor industrial-scale case studies were found in the literature; they were only highlighted as future works. Although the NB-stabilizing effects of electrolytes have been well-explored, the mechanisms related to surfactants remain the issue of further investigation. The effectiveness of the NB-assisted flotation processes has been mostly addressed for single minerals, and only a few works have been reported for bulk materials. Finally, we believe that the current review paves the way for an appropriate selection of generating and detecting ultrafine bubbles and shines the light on a profound understanding of its effectiveness.

Nanobubble boundary layer thickness quantified by solvent relaxation NMR

HYPOTHESIS

The boundary layer holds the key to solve the puzzle of the unusual stability of the nanobubbles in solution. The quantitative determination on its mechanical and structural properties has not been achieved due to its diffusive and dynamic nature, lack of distinctive interfaces, and difficult differentiation from bulk background. Therefore, it is necessary to investigate this boundary using more sensitive interface analysis technologies to effectively differentiate the water molecules at the interface from those in the bulk.

EXPERIMENTS

An in-situ and non-deconstructive method, solvent relaxation nuclear magnetic resonance, was used to investigate the boundary layer on bulk nanobubbles, where the relaxation rate of the water in the layer and its thickness were measured by solvent relaxation NMR and the ratio between the water molecules at the bubble interfaces and those in the bulk and the corresponding boundary layer thickness were determined.

FINDINGS

The spin-spin relaxation time for the water in the layer (∼10¹ms) is found to be two orders of magnitude lower than that of the free water (∼10³ms). As the first attempt, the determined boundary layer thickness is around 35-45 nm and 17.0 %-8.7 % of the effective gaseous size of the nanobubbles, which increases with the decrease of the bubble diameter. As a result, a quantitative measurement model for bubble boundary layer has been established in order to better understand the interfacial properties and stabilization mechanism for bulk nanobubbles.

Generation of Bulk Nanobubbles by Self-Developed Venturi-Type Circulation Hydrodynamic Cavitation Device

Bulk nanobubbles (BNBs) have attracted substantial interest from academia and industry owing to their peculiar properties and extensive potential applications. However, a scalable engineering method needs to be developed. Herein, we developed a nanobubble generator based on venturi-type recirculating hydrodynamic cavitation. The existence of nanobubbles produced by our generator was confirmed using physicochemical test methods, including the Tyndall effect, multiple freeze-thaw degassing experiments, and trace metal analysis. Subsequently, the effects of different operating parameters (circulation time and operating pressure) on bulk nanobubble production and properties, as well as their stability, were investigated. The results suggest that the characteristics of BNBs varied with the circulation time (5-20 min) and operating pressure (2-5 bar). However, all the particle size distribution of BNBs had a bimodal distribution with a mean diameter of 180-210 nm for the different circulation time and operating pressures. For example, by increasing the circulation time from 5 to 20 min, the peak value of size distribution decreased from 333/122 nm to 218/52 nm, and the average sample scattering signal count rate (Avg. Count Rate) increased from 133 to 303 Kcps. The evaluation of the stability of the BNBs formed for the circulation time of 15 min and the operating pressure of 3 bar showed that they could continue existence and stability in the suspension for 72 h. The study results might provide a valuable method for further investigation of industrial applications of venturi-type nanobubble generators.

Can bulk nanobubbles be stabilized by electrostatic interaction?

It has been suggested that electrostatic stress arising from charges accumulated at the surface of nanobubbles might balance Laplace pressure leading to their stability. This mechanism has been widely discussed in the nanobubble field for the past decade. However, the stress in the diffusive double layer was overlooked when calculating the electrostatic effect in previous theories. In this communication, we recalculated this effect using the classical double layer theory. Combined with experimentally measured zeta potential, we find that the ratio of electrostatic pressure to Laplace pressure is much less than 10-2, which suggests that electrostatic interaction may not be the main factor for stabilizing bulk nanobubbles.